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ENSO Quick
Look IRI ENSO Update / Forecast IRI Probabilistic ENSO Forecast Technical ENSO Update ENSO Prediction Plume Monthly archive Technical ENSO Update16 August 2007
Current ConditionsAs of mid-August 2007 SSTs in the eastern equatorial Pacific are below average, and their longitudinal extent has tended to increase since their initial appearance in February and March, despite some month-to-month fluctuations. The western flank of the region of below average anomalies has expanded westward to approximately the dateline, although anomalies between 150W and the dateline remain weak. Easterly wind anomalies have been weak but consistent since March, with the exception of an interruption in late May. Even with this break, the mainly easterly anomalies over the last five months have induced a tendency toward shoaling of the thermocline, which continue currently in the central Pacific, overlaid with weaker negative anomalies in the mixed layer. The tendency toward shallow thermocline anomalies since February or March 2007 is indicative of the slowly evolving movement toward a La Niņa condition the equatorial Pacific.For July 2007, the SSTs in the NINO3.4 region were slightly below average (-0.3 degrees C anomaly), and for the May-Jun-Jul season were -0.14 degrees C from average. Currently the IRI's definition of El Niņo conditions rests on an index of SST anomalies, averaged over the NINO3.4 region (5S-5N; 170W-120W), exceeding the warmest 25%-ile of the historical distribution, and similarly for La Niņa relative to the 25%-ile coldest conditions in the historical distribution. The magnitudes of NINO3.4 anomaly necessary to qualify as La Niņa or El Niņo conditions for the Sep-Oct-Nov and Jul-Aug-Sep seasons are approximately 0.55C and 0.70C, respectively, as interannual variability increases in the latter portion of the calendar year. Expected ConditionsThe current SST anomaly in the NINO3.4 region, at -0.5 to -0.6C, is at the borderline of qualifying for a La Niņa condition. The issue at this point is whether persistence, or further amplification, of this condition is likely. Factors of relevance are the shallow thermocline anomalies extending across much of the equatorial Pacific over the last several months, the negative SST anomalies in the eastern and now east-central equatorial Pacific, and persistence of the general enhancement of easterly Trade Winds across much of the central and eastern tropical Pacific despite a significant interruption on a shorter time-scale in June due to a strong intraseasonal event.Many of the features in the tropical Pacific air-sea system are consistent with the development of La Niņa conditions -- shallow thermocline in the east and east-central longitudes, cold eastern and more recently east-central Pacific SST anomalies, and easterly wind anomalies. However, some uncertainty nonetheless remains concerning the duration and magnitude of the episode whose onset appears to be taking place presently. Continuation of enhanced easterly Trade Winds will be required to draw the existing supply of subsurface negative temperature anomalies to the surface and increase/maintain coupled air-sea growth. The strength and persistence of the Trade Wind anomalies, and subsequent strengthening of convection anomalies, in coming several months are likely to determine whether this event will remain at a borderline or minimal level, or develop into an episode of weak to moderate intensity. Current evolution of the air-sea system looks indicative of at least weak La Niņa development. Given the relative persistence of the coupled air-sea system over the last several months, there is a fairly good probability that the system could continue as weak La Niņa conditions over the coming seasons. Thus, a 60% probability of at least weak weak La Niņa conditions is suggested. Development of El Niņo conditions is very unlikely,
with probabilities not exceeding 10% through the Feb-Mar-Apr
of 2008. There is an
approximately 60% likelihood for La Niņa conditions during the
Aug-Sep-Oct season, and a 39% probability
for ENSO-neutral conditions. The probabilities for La
Niņa stay over 50%, but decline slowly, toward the end of 2007. The above assessment was made in part on the basis of an examination of the current forecasts of ENSO prediction models as well as the observed conditions. For purposes of this discussion, El Niņo SST conditions are defined as SSTs in the NINO3.4 region being in the warmest 25% of their climatological distribution for the 3-month period in question over the 1950-present timeframe. The corresponding cutoff in terms of degrees C of SST anomaly varies seasonally, being close to 0.45 degrees C in northern late-spring/early-summer season and as high as 0.75 degrees C in late northern autumn. La Niņa conditions are defined as NINO3.4 region SSTs being in the coolest 25% of the climatological distribution. Neutral conditions occupy the remaining 50% of the distribution. These definitions were developed such that the most commonly accepted El Niņo and La Niņa episodes are reproduced. There is some variation among ENSO model forecasts through
the 10-month forecast period. Over the coming several months, slightly
more than half of the predictions indicate weak to moderate La Niņa
conditions; the rest predict ENSO-neutral conditions. Just one
model predicts El Niņo conditions beginning shortly after the beginning
of 2008. For the ASO 2007
season, 11 of 21 models (52%) predict La
Niņa conditions to exist. At lead times of more than 4
months into the future, statistical and dynamical models that
incorporate information about the ocean's observed sub-surface thermal
structure generally exhibit higher predictive skill than those that do
not. Among models that do use sub-surface temperature information, 6 of
13 (46%) indicate La Niņa conditions will have continued into
the Dec-Jan-Feb season, and 7 of 13 (54%) predict
that SSTs will be ENSO-neutral (Note
1). Caution is
advised in interpreting the distribution of model forecasts as the
actual probabilities. At longer leads, the skill of the models
degrades, and skill uncertainty must be convolved with the
uncertainties from initial conditions and differing model physics,
leading to more climatological probabilities in the long-lead ENSO
Outlook than might be suggested by the suite of models.
Furthermore, the expected skill
of one model versus another has not been established using uniform
validation procedures, which may cause a difference in the true
probability distribution from that taken verbatim from the raw
model predictions. An alternative way to assess the probabilities of the three
possible ENSO conditions is to use the mean of the forecasts of all
models, and to construct a standard error function centered on that
mean. The standard error would be Gaussian in shape, and would have its
width determined by an estimate of overall expected model skill for the
season of the year and the lead time. Higher skill would result in a
relatively narrower error distribution, while low skill would result in
an error distribution with width approaching that of the historical
observed distribution. When this method is applied to the current model
forecasts, results indicate similar probabilities for La Niņa,
ranging from 60% likely in the Aug-Sep-Oct season,
dropping to just below 40% likely by the beginning of 2008. This method
shows probabilities for ENSO-neutral beginning slightly below the
climatologically expected 50%, rising to near or slightly above 60%
by the beginning of 2008. The same cautions mentioned above for the
distribution of model forecasts apply to this alternative method of inferring
probabilities, due to differing model biases and skills. In particular,
this approach considers only the mean of the predictions not their
range. The IRI's
probabilistic ENSO forecast takes into account the indications of
this set of models, the outcome of the standard error approach
described above, and additional factors such as the very latest
observations that may have developed after the initialization times of
some of the models. It indicates that a 55% probability for La
Niņa
development in the coming season. See also: Note 1 - Only models that produce a new ENSO forecast every month are included in the above statement.
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